1. Field of the Invention
This invention generally relates to Anti-Reflective Coatings (ARCs) for multi-layer i-line photoresist systems. More particularly, it relates to improved i-line dyes, which dyes have differential solubility characteristics for use in ultra-thin layer ARCs.
2. Background of the Prior Art
Recently, integrated circuit (IC) manufacturers when printing submicron feature sizes in the range of 0.35-0.70 microns have relied upon mid-ultraviolet (mid-UV) sensitive photoresist. The principal exposure wavelength for these photoresists is 365 nm, which corresponds to the i-line spectral peak of the Hg--Xe arc lamp. The use of this shorter wavelength, as compared to the 436 nm g-line peak, permits greater resolution and broadens depth of focus. Thus, the term "i-line resist" has been used inter-changeably with "mid-uv photoresist". The resolution of i-line resists can be further enhanced by providing substrate reflection control. This can be achieved in two ways. The first way is by adding dye into the resist at some expense to photospeed and feature profile quality. The second way is by employing a multi-layer resist scheme wherein a dyed bottom layer plays the light absorbing role.
Two types of multi-layer schemes have been used in the industry. At one time, relatively thick (ca. 1 micron) bottom layers having low dye concentration were popular. It is desirable that those bottom layers now be replaced by high optical density, ultra-thin ARCs to simplify the etching of the ARC, which would lead to improved critical dimension control. However, the development of ultra-thin ARCs for i-line applications has been hampered by the unavailability of suitable dyes.
Some advances have been made in developing prior art i-line dyes for single-layer-resist-type processes. However, there is a drawback to such dyes, as well as dyes designed for the multi-layer resist processes, which accommodate a relatively thick (1.3 micron) bottom or sublayer ARC. This drawback of the prior art and their attendant i-line dyes are explained below.
An example of a classically cited strongly absorbant mid-UV i-line dye in the prior art is Michler's ketone (bis-N,N-dimethylaminobenzophenone). In order to employ Michler's Ketone in photo-etch processing, conditions peculiar to such processing must be accommodated. That is, it is common to process photoresist layers and their ARCs at temperatures as high as 200.degree. C. Unfortunately, Michler's Ketone, like many other prospective i-line dyes, tends to sublime or volatize at such temperatures, and therefore lacks the requisite thermal stability to undergo such processing.
In U.S. Pat. No. 4,719,166, R. W. Blevins disclosed a single layer non-subliming resist system which used butadienyls as i-line dyes. The dyes surprisingly overcame the tendency of previously known mid-UV dyes to sublime at processing temperatures near 200.degree. C., even though the molecular weight of the butadienyls was less than the molecular weight which would have predicted thermal stability. The predictably higher molecular weight dyes, as taught by Blevins, were to be avoided because they would not exhibit good solubility in photoresist solvents such as chlorobenzene and ethoxyethyl acetate. A drawback of these dyes is that the process used to produce the butadienyls yields the butadienyls at an unacceptably low 10%. More importantly, these dyes are so soluble that they cannot be used in ultra-thin ARCs without leaching into the photoresist top layer of multi-layer systems.
In U.S. Pat. No. 5,139,918, Mr. A. Goel disclosed a multi-layer resist system which employed butadiene or bromine substituted butadienes as the i-line dye, especially N,N'-dibutyl-N,N'-di(1-(4,4-dicyano-1,3-butadiene))-1,6-hexanediamine. Although this dye system is acceptable for the relatively thick (1.3 micron) bottom layers of the multi-layer resist systems, it is ineffective for ultra-thin ARCs. Like the single layer i-line dyes, these butadienes are highly soluble in photoresist solvent systems. Therefore, although the 1.3 micron thickness of the bottom layer ARC serves to lessen the leaching, ultra-thin ARCs suffer so much loss of dye that substrate reflection cannot be controlled.
The drawback to each of these prior art i-line dyes is that they lack effective differential solubility. That is, the very same solubility in photoresist solvents such as chlorobenzene and ethyoxyethyl acetate which allow the dyes to be formulated in known polymer coating systems such as polymethyl methacrylate (PMMA), will either limit dye use to single layer photoresist systems, or risk, in multi-layer systems, having the dye leaching out of the ARC layer into the top photoresist layer.
Accordingly, a non-subliming i-line dye having strong mid-UV absorptivity which would be sufficiently soluble in ultra-thin ARC layers to produce an effective high optical density in layers having less that 0.25 microns thicknesses, yet characterized by being insoluble in photoresist solvents, would represent an unexpected advancement in the art and fulfill a long felt need in the industry.